28 Rev. A. Sedgwick and Mr. Murchison on the Geological 



stone, when in immediate contact with the trap ; and these are again under- 

 laid by thin beds of shale with obscure impressions of plants, and by beds of 

 sandstone. The altered carbonaceous shale frequently breaks into rhomboidal 

 fra"ments and has associated with it thin laminae of a substance resembling 

 anthracite, probably derived from the vegetable remains which accompany 

 these beds. Such changes, however, are not uniform; for other beds, pos- 

 sessing the ordinary soft shaly texture, are to be seen in absolute contact with 

 the trap. The lowest shales rest upon, and alternate with, very hard light- 

 coloured sandstone, which may be considered as terminating the carboniferous 

 group. 



Old Red Safidstone. 



The line of demarcation between this formation and that just described 

 must be regarded as in some degree arbitrary ; since in following the line of 

 coast for several hundred feet, the dip undergoes no change, and the physical 

 character of the beds continues nearly the same, except that vegetable fossils 

 are now absent. 



On approaching Groggan Point, the beds are much changed in structure : 

 highly indurated sandstone of a light red tinge alternates with masses of in- 

 durated shale of a red, gray, and greenish gray colour. Associated with the 

 above are hard dark-coloured calcareous beds, containing many quartz pebbles, 

 thus passing into a conglomerate form; and subordinate to the above, are many 

 irregular concretions of nearly compact carbonate of lime not to be distin- 

 guished from the cornstone of Herefordshire. These characters, and the ab- 

 sence of vegetable fossils, induced us to separate this group from the carboni- 

 ferous series, and to place it at the head of this section. At Groggan point 

 are seen the first beds of unequivocal old red sandstone gradually passing from 

 a pink-coloured grit into a very coarse conglomerate, which rising high upon 

 the flank of the mountain, increases in the angle of dip, viz. from 45° to 70°, in 

 proportion as it recedes from the shore. Here the schistose mountains ap- 

 proach rather abruptly towards the coast, but they never, as has been some- 

 times stated, entirely cut off" the zone of secondary rocks we are now de- 

 scribing*. 



In the adjoining small bay, white sandstone and purple-coloured shale are 

 succeeded by a blood-coloured spotted sandstone, from beneath which a vast 

 hill of conglomerate called Craig a Caajou (cheese crag) rises to a height of 

 from eight to nine hundred feet. Here there is a most remarkable debacle of 

 the conglomerate, enormous masses of which have fallen from the precipitous 



* See Headrick on the Isle of Arran, p. 221, &c. 



